1. Technical Field
The present disclosure pertains to the generation of light and, more particularly, to a light source system that generates stable optical power over time and temperature.
2. Description of the Related Art
Constant-intensity light sources are commonly used in industrial applications where the light intensity has to remain unchanged under varying ambient conditions, such as temperature, pressure, and humidity. Since the optical and electrical properties of all light-emitting semiconductors and optical materials vary with temperature and other ambient conditions, an absolutely stable light source does not exist. Instead, conventional sources of varying stability are developed for specific applications.
Attributes of light sources that are critical for specific applications may include wavelength, line width, stability, power consumption, size and cost. When size and cost are the primary concerns, semiconductor lasers and light-emitting diodes are the semiconductor light emitters of choice, and if simple beam-forming measures are required to confine the output light into a group of parallel rays, vertical-cavity surface-emitting lasers (VCSELs) and surface-emitting resonant-cavity light-emitting diodes (RCLEDs) are particularly well suited for many applications because they have chip sizes smaller than 0.1 mm2 and emit round conical beams of visible (RCLEDs and VCSELs) and near-infrared light (VCSELs). Furthermore, micro-lenses and reflectors are commercially available and in some cases can be integrated directly with RCLEDs and VCSELs. Producing stable light intensity when temperature changes during operation, however, places challenging demands on the design and generally increases the size and the cost of a device. For this reason, light-source designers usually must sacrifice high stability for small size to create light sources for a particular application.
Optical sources used in fiber-optic communication systems and optical sensing typically require optical power levels that can vary by as much as 15% over 50° C. This level of control can be achieved using a sensor placed close to the semiconductor light emitter to measure its temperature and an electronic circuit that varies the drive current through a semiconductor light emitter to maintain nearly constant output light intensity. For example, see U.S. Pat. No. 4,841,157. The main reason why this type of output control yields inadequate light source stability for many applications is that the output intensities of semiconductor light emitters typically used for this purpose decline with time at unpredictable and uncontrollable rates.
In another conventional system, a fraction of the light emitted by the semiconductor light emitter is compared to a preset signal, and the light-emitter drive current is continuously adjusted electronically to bring the output intensity to a desired value. This approach is common in nearly all fiber-optic modules and light sources in conventional sensing applications and results in light-intensity fluctuations less than 5% over 50° C. Examples of such designs are found in U.S. Pat. Nos. 5,209,112; 5,796,481; 6,222,202; and 6,483,862. While photodetectors have been integrated within VCSEL chips as described by Grabherr et al. 2009 (Grabherr, M., R., Phillip Gerlach, Roger King and Roland Jäger. 2009. Integrated photodiodes complement the VCSEL platform. Proceedings of SPIE Vol. 7609-03.), the control accuracy of these detectors in the aforementioned systems is and will continue to be unsatisfactory for the present disclosed system.
A light-measurement system having elements, detectors, and algorithms to determine the wavelength of incoherent, unpolarized light is described in U.S. Pat. No. 5,515,169 (Cargill et al.). Although the polarization properties of the analyzed light and the management thereof are unimportant in that system, the sensitivity of the linear wavelength filter to angle of incidence and polarization is problematic in the present disclosure and precludes use of the prior methods. In related technology, U.S. Pat. No. 6,128,133 (Bergmann) describes a BK7 glass plate beamsplitter with first and second surfaces with optical coatings that reflect orthogonal polarizations that do not interfere with one another. The inventors further disclose in FIGS. 3A and 3B the extinction ratio of the coatings to be on the order of 30 dB (1000:1). The extinction ratio of a beamsplitter equals the maximum transmittance divided by the minimum transmittance.
A system that requires a ratio greater than 40 dB (10,000:1 with coating that splits and focuses an input beam onto two spatially separated spots is found in U.S. Pat. No. 5,657,164 (Shuman) for use in optical disc reading and writing with a semiconductor chip with integrated light emitter and detector. Moreover, the nonparallelism of the first and second plate surfaces can be as much as 2° of arc, and the coatings are sensitive to angle of incidence. The angle of incidence is defined as the angle between the direction of the incident light beam and the direction normal to the surface upon which the beam is incident. Both characteristics make the prior approaches useless wherein the second surface is made substantially transparent by normal incidence and the use of an AR coating, neither of which are disclosed in U.S. Pat. No. 6,128,133.
Certain applications, including but not limited to laser scanning, water- and air-turbidity sensing, airborne-particle analysis, fog and visibility monitoring and blood-gas analysis require light sources with output intensities that change less ½% over a 50° C. range for various absorption, transmittance and light-scattering measurements would significantly benefit from miniature highly stable light sources. A miniature light source is defined as a light source that can be driven from two AA alkaline batteries and is about the size of an ordinary marble (roughly 1 cm3, or a JEDEC TO-5 package). These requirements exclude the implementation of thermoelectric coolers to maintain the temperature and wavelength at pre-set values because they consume a large amount of power. Presently there is no such product on the market, while clearly there is a need in the industry for such a miniature light source that remains stable under ambient temperature variation.